Astrophysics

The Local Standard of Rest

“Whereas all the planets, asteroids and meteors that originate within the solar system more or less circle what is called the Ecliptic plane, that of our sun, since they were formed from the same disc of gas and dust that rotated around itself, Oumuamua entered the solar system north of the plane, in an extreme hyperbolic orbit and at a speed of 26.3 kilometers per second faster relative to the motion of the sun.

“A reconstruction of its trajectory shows that Oumuamua traversed the ecliptic plane on September 6, 2017, when the sun’s gravity accelerated the object to a velocity of 87.8 kilometers per second. On September 9, the object passed closer to the sun than the orbit of Mercury. And on October 14, five days before it was discovered in Hawaii, the object passed 24.18 million kilometers away from Earth, or 62 times the distance from here to the moon.

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“I wrote above that Oumuamua originated at Vega, but that’s not completely accurate: The universe is a vast place, and even at Oumuamua’s velocity – a velocity that no human spaceship has achieved – a voyage from Vega to the solar system would take 600,000 years. But in the meantime, Vega is orbiting the center of the Milky Way, like the sun and all the other stars, and it wasn’t in that region of the heavens 600,000 years ago.

“If you average the velocities of all the stars in the region, you get a system that’s called the ‘local standard of rest.’ Oumuamua was at rest relative to that system. It didn’t come to us. It waited in place, like a buoy on the surface of the ocean, until the ‘ship’ of the solar system ran into it. To make things clear, only one of 500 stars in the system is as much at rest as Oumuamua. The probability of that is very low. After all, if it were a stone that was simply hurled from a different solar system, we would expect it to have the velocity of its star system, not the average velocity of all the thousands of stars in the vicinity.”

Text: “If True, This Could Be One of the Greatest Discoveries in Human History”, Haaretz.

Image: Augsburger Wunderzeichenbuch, Comet mit einem grosen Schwantz, 1401

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Allowed & Forbidden Universes

“On June 25, Timm Wrase awoke in Vienna and groggily scrolled through an online repository of newly posted physics papers. One title startled him into full consciousness.

“The paper, by the prominent string theorist Cumrun Vafa of Harvard and his collaborators, conjectured a simple formula dictating which kinds of universes are allowed to exist and which are forbidden, according to string theory. The leading candidate for a “theory of everything” weaving the force of gravity together with quantum physics, string theory defines all matter and forces as vibrations of tiny strands of energy. The theory permits some 10,500 different solutions: a vast, varied “landscape” of possible universes. String theorists like Wrase and Vafa have strived for years to place our particular universe somewhere in this landscape of possibilities.

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“But now, Vafa and his colleagues were conjecturing that in the string landscape, universes like ours—or what ours is thought to be like—don’t exist. If the conjecture is correct, Wrase and other string theorists immediately realized, the cosmos must either be profoundly different than previously supposed or string theory must be wrong.

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“The conjectured formula—posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri, and Lev Spodyneiko, and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal, and Paul Steinhardt—says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string-theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.

“Since 1998, telescope observations have indicated that the cosmos is expanding ever so slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”

“In addition, it looks like the amount of dark energy infused in empty space stays constant over time (as best as anyone can tell).

“But the new conjecture asserts that the vacuum energy of the universe must be decreasing…”

Text: The Universe as We Understand It May Be Impossible, The Atlantic.

Pic: Oystein Aspelund, from Dark Brasilia